One present method of growing plants is described in U.S. Pat. No. 4,279,101 issued July 21, 1981 to Leroux; it consists of spreading soil over the bottom of a tray and roots are indinstinctly plunged into the soil. This method leads expectedly to cross-linkage between roots as plants grow; consequently, if some of the plants are removed from the tray, there is a risk of damaging several roots, either that of the remaining plants or of those being removed. Another method of plant growing is described in U.S. Pat. No. 4,251,951 issued February, 1981; it consists of cultivating plants in slabs mounted on supported frames equipped with supporting rails. With this method, cross-linkage between roots is prevented; however, the stems are directly exposed to the surroundings and, during transportation of the tray, great care must be taken to avoid damaging the stems.